A computerized tomography (CT) imaging apparatus operates by acquiring multiple 2D images with a rotating imaging ensemble or gantry that has an x-ray source and, opposite the x-ray source, an imaging sensor rotating about a fixed axis relative to the patient. CT imaging allows the reconstruction of 3D or volume images of anatomical structures of the patient and is acknowledged to be of particular value for obtaining useful information for assisting diagnosis and treatment.
There is considerable interest in the use of CT imaging in dental and ear-nose-throat (ENT) applications, as well as for other imaging of the patient's head. A number of volume imaging system designs have been proposed for this purpose. Among proposed solutions are hybrid systems that combine panoramic imaging and CT imaging. For example, U.S. Pat. No. 6,118,842 entitled “X-RAY IMAGING APPARATUS” to Arai et al. discloses an X-ray imaging apparatus that supports both imaging modes. The apparatus includes an X-ray source, an X-ray detector for detecting X-rays having passed through the subject, and supporting means for supporting the X-ray source and the X-ray detector so that they are spatially opposed to each other across the subject; and mode switching means for switching between a CT mode and a panorama mode. To detect X-rays, only one large area X-ray detector is used. The X-ray imaging apparatus can obtain both types of images by switching modes during the imaging session. However, the proposed imaging apparatus requires an expensive detector capable of carrying out both imaging functions in a satisfactory manner. Additionally, systems of this type typically compromise image quality by using a uniform distance between the X-ray source and detector, even though different distances would be more advantageous.
By way of example, FIG. 1 shows an embodiment of a conventional CT imaging apparatus 40. A column 18 is adjustable for height of the subject. The patient 12 or other subject, shown in dotted outline, is positioned between an x-ray source 10 and an x-ray imaging sensor panel 20, also termed an imaging detector. X-ray imaging sensor panel 20 rotates on a rotatable mount 30 in order to position a CT sensor 21 for obtaining the exposure. CT sensor 21 is positioned behind the subject, relative to x-ray source 10. The operator rotates CT sensor 21 into this position as part of imaging setup. With rotation of mount 30, sensor 21 and source 10 revolve about the head of the patient, typically for some portion of a full revolution. Still other dental imaging system solutions combine CT, panoramic, and cephalometric imaging from a single apparatus. With such combined systems, the required amounts of radiation exposure can be a concern, particularly for CT imaging, which can require numerous images, each from a separate exposure.
Conventional digital radiography detectors have some limitations related to how attenuation of radiation energy at a single exposure is interpreted. For example, it can be very difficult, from a single exposure, to distinguish whether an imaged object has a given thickness or a given attenuation coefficient. To resolve this ambiguity, some systems provide separate, sequential low-energy and higher energy exposures and use the resulting difference in image information to distinguish between types of materials. However, in order to provide this information, this type of imaging requires that the patient be subjected to additional radiation for the second exposure. This problem can be compounded for CT imaging, in which multiple images are obtained, one from each of a number of angles of revolution about the patient.
Computed tomography (CT) and cone beam computed tomography (CBCT) systems reconstruct volume image data from a series of 2D x-ray images, termed “projection images”, obtained at different angular positions about the imaged subject. An iterative reconstruction method is employed to use data from the 2D images for this purpose.
Cone beam scanners generally use polychromatic X-ray sources because of their lower cost and availability as compared with monochromatic X-ray sources which either require a synchrotron or an X-ray monochromator. The broad-spectrum radiation that is emitted from the polychromatic X-ray source is attenuated by the material that is being imaged, according to its x-ray attenuation coefficient, which varies with the type of material.
Among the problems encountered in obtaining image data for accurate 3D reconstruction is beam hardening. Beam hardening occurs as the polychromatic or polyenergetic radiation progresses through the subject material. Energy of different wavelengths is absorbed at different rates, according to the irradiated subject material. As a result of energy absorption of particular wavelengths by the material, the energy spectrum of the polychromatic X-ray radiation varies with location or depth in the scanned object and this variation depends on both the spatial characteristics or depth of the object and the relative location of the X-ray source. Because lower-energy radiation (at lower frequencies or longer wavelengths) is attenuated more strongly than higher-energy radiation (at higher frequencies or shorter wavelengths), the radiation beam is “hardened”. For a uniform cylindrical phantom, for example, X rays passing through the middle portion of the phantom pass through more material than X-rays passing through edge portions. As the X-ray energy encounters more material, its spectral content changes and is considered to be more “hardened” than the same energy directed through less material; the proportion of higher energy to lower energy increases as the radiation travels further through the object. From a spectral aspect, the energy spectrum changes along the beam path that the radiation follows through the material, even where the object is of uniform depth and material composition. This change in the spectral content of the beam causes artifacts such as cupping, in which the middle of the subject experiences different radiation levels than portions of the edge of the subject. These beam hardening artifacts can appear as dark bands between highly attenuating parts of the imaged object.
Beam hardening complicates the task of 3D image reconstruction in CBCT and other volume imaging modalities. The 2D image content that is used to reconstruct a particular 3D voxel can be affected differently according to the angle at which the 2D image is obtained and the location of the voxel within the imaged object. Thus, there is a need for image processing methods that compensate for beam hardening in 2D images and in 3D image reconstruction.
In conventional CBCT volume reconstruction, the volume image that is generated provides only a single data value for each voxel, according to the total amount of attenuation measured at each position within the object. This single data value is not sufficient for determining the material composition at that voxel; only a rough guess of the material combination can be made. It would be of particular value to be able to obtain additional information for each voxel. Attenuation coefficients at two or more different energy levels, for example, would provide sufficient information to allow a more accurate estimate of the material composition of the reconstructed data.